The present invention relates to expandable polyolefin resin compositions, and more particularly to expandable polyolefin resin compositions suitable for preparing crosslinked polyolefin resin foams which have high heat resistance and can be fabricated into complex shapes. The invention further relates to cross-linked polyolefin resin foams prepared from such resin compositions by crosslinking and subsequent foaming.
Polyolefin resin foams are in prevalent use as heat-insulating materials and miscellaneous goods because of their excellent flexibility and high heat resistance, and have recently found wide use as interior finishing materials for motor vehicles and like vehicles, for example, as heat-insulating materials for ceilings, doors, instrument panels, etc. Such interior finishing materials for vehicles are prepared, for example, by affixing a sheet of crosslinked polyolefin resin foam to a skin material and forming the resulting shape by vacuum forming or compression molding.
Generally, vacuum forming or compression molding is conducted at a high temperature of 120.degree. to 200.degree. C. For molding at such a high temperature, conventional polyethylene-base polyolefin resin foams are low in heat resistance and therefore unusable, and polypropylene-base polyolefin resin foams are used.
In producing polyolefin resin foams, resin compositions are crosslinked usually by 1) heating the resin composition with an organic peroxide incorporated therein, or 2) irradiating the resin composition with an ionizing radiation. These methods are practiced industrially. However, although effective for cross-linking polyethylene, these crosslinking methods are not suited to the above-mentioned polypropylene-base polyolefin resin because when crosslinked by either of these methods, polypropylene has its molecular chain broken at the same time to deteriorate. In view of this fact, polypropylene-base resins are crosslinked by admixing a polyfunctional monomer, such as divinylbenzene, diethylene glycol dimethacrylate or trimethylolpropane tri(meth)acrylate, with the resin and irradiating the resulting resin composition with an ionizing radiation (Examined Japanese Patent Publications SHO 46-38716 and SHO 60-11935, Unexamined Japanese Patent Publications SHO 57-212236 and SHO 61-69844, etc.). Among these publications, the publication SHO 46-38716 proposes two kinds of auxiliary crosslinking agents: a) a compound having at least two vinyl-type double bonds in the molecule, and b) the combination of an aromatic compound having one vinyl group in the molecule and an organic compound having at least two (meth)acryloyloxy groups in the molecule. The latter is favorably used chiefly for crosslinking resins with an organic peroxide.
Nevertheless, when these polyfunctional monomers are used for crosslinking resin compositions, the polyfunctional monomer, which has different compatibilities with different resin components of the composition, becomes present as localized in the resin component having the highest compatibility therewith, permitting the composition to be crosslinked unevenly when irradiated with an ionizing radiation.
Further when polyolefin resin compositions are given an increased gel fraction to obtain a viscosity required for foaming in such a state, the resin foam obtained has higher hardness and less elongation than is needed for molding. Difficulties are then encountered in molding the foam into a complex shape having a deep recess, possibly causing the rupture of the foam. If a reduced amount of polyfunctional monomer is used to improve the moldability, the composition can not be crosslinked uniformly, failing to give a foam which appears homogeneous.
In view of the foregoing problems, an object of the present invention is to provide an expandable polyolefin resin composition which can be crosslinked uniformly in its entirety to produce a crosslinked polyolefin resin foam which is excellent in heat resistance, toughness and moldability or formability and which appears homogeneous.
Another object of the invention is to provide a crosslinked polyolefin resin foam which is obtained by crosslinking and foaming the expandable polyolefin resin composition.